APOPTOSIS (PROGRAMMED CELL-DEATH) AND OTHER REASONS FOR ELIMINATION OF NEURONS AND AXONS

Many of the neurons which migrate to the cortex die at an early stage of development, In certain animals many cells die when their task is d one; a phenomenon known as programmed cell death, or apoptosis, In hum ans a far commoner cause is the competition hypothesis: if the axon of the cell does not make contact with the dendrites of the cell in its target area it will die, Then the complexity and number of these synap ses is more essential to the function of the brain than the actual num ber of neurons. Examples are given of the effect of sensory deprivatio n on the survival of neurons, The withdrawal of neuronal growth factor , and subsequent loss of the axon or cell can be a factor in the fine tuning of neural circuits, and therefore in an improvement in learning and the development of skills, However, it may also account for the d iminished plasticity of the nervous system, and its ability to adapt t o injury. It has been postulated that these changes contribute to the development of personality. Factors which may be involved include diff erences in brain development between the sexes; temperamental traits w hich may have a genetic or prenatal origin; and the effects of sensory perception, including language. Surely there can be no doubt that the results of experience represent the fundamental mechanisms by which t he organism adapts to its environment.